This invention relates to resin-containing cellulosic overlays useful for masking surface imperfections of woody substrates and improving the paintability, abrasion resistance and weathering properties thereof.
Such overlays are well known in the art and are generally prepared using one of two methods. In one method a liquid resin usually comprised of low molecular weight polymers is applied to a preformed paper sheet, such as by immersing the sheet in a saturating or impregnating resin bath. The sheet is then dried prior to application to a substrate. This invention does not relate to overlays produced by this method. Rather, it relates to a method commonly referred to as the "beater-addition method." In this method, an aqueous, alkaline solution of a resole resin (typically a phenol-formaldehyde resin) is added to an aqueous slurry of cellulosic fiber. The resulting furnish is thoroughly mixed to obtain uniform resin distribution and the resin is then precipitated onto the fibers by acidification of the slurry. The slurry is then converted into a wet sheet using conventional paper-making apparatus, the wet sheet then being dried. The resulting hard-surfaced, flexible, tough and substantially opaque sheet can be applied to plywood, lumber, particle board or similar woody substrates using one of many commercially available adhesives useful for this purpose. Typically, heat and pressure are applied to the substrate-overlay laminate to assure complete bonding therebetween and to cure the resin in the overlay.
Panels and boards surfaced with overlays prepared by the beater-addition method have found a wide range of applications where smooth, uniform, substantially opaque surfaces that will resist weathering and abrasion and readily accept paint are required. Among these applications are siding, soffit and panel applications in buildings; traffic signs; planking, bulkheads and transoms on boats; and concrete forms.
One shortcoming of prior art beater-addition overlays is that they tend to yellow during hot pressing and continue to develop color with age (i.e., exposure to light and air). This has prevented such overlays from being used in applications where white color is important, such as where they are overprinted with various designs such as those simulating light colored wood.
It is an object of this invention to provide beater-addition overlays that are white in color and are color stable upon aging and upon exposure to caustic.